Our studies have shown that veratrine in low doses increases the threshold for electrically induced ventricular fibrillation in the dog heart. Preliminary observations indicate that a similar effect takes place during induced myocardial ischemia. These effects are associated with an increase in the atrial and ventricular refractory periods, but are not associated with significant effects on A-V nodal refractoriness or conduction time. Ventricular Purkinje fibers, and to a lesser degree ventricular muscle fibers, show a prolongation in the action potential when exposed to veratrine. In the proposed work we will complete our studies on the electrical stabilizing effect of veratrine in the ischemic dog heart. Second, using the micro-electrode technique, we will complete our studies on the effects of veratrine on excitability, membrane responsiveness, and conduction in isolated preparations of the canine ventricular conduction system. We hope to gain further insight into the electrophysiologic effects of veratrum alkaloids and elucidate the mode of their antiarrhythmic effects. BIBLIOGRAPHIC REFERENCES: Arbel, E.R., Ellis, A., Barzilai, B. and Glick, G.: Protective effect of veratrine against ventricular fibrillation in the dog. (Abstract) Clin. Res. 23: 379A, l975. Cohen, H.C. and Arbel, E.R.: Tachycardias and electrical pacing. Med. Clin. N. Am. 60: 343-367, l976.